PayPal\Api\Invoice::addBillingInfo PHP Method

    public function addBillingInfo($billingInfo)
    {
        if (!$this->getBillingInfo()) {
            return $this->setBillingInfo(array($billingInfo));
        } else {
            return $this->setBillingInfo(array_merge($this->getBillingInfo(), array($billingInfo)));
        }
    }